This invention relates to a method of methanol production having reduced emission of carbon dioxide.
Methanol is a synthetic fuel which is produced from reactants which provide carbon, hydrogen and oxygen. There are various sources of each of these molecules. For example, the requisite carbon may be obtained from coal (see for example U.S. Pat. No. 4,476,249 Avery), natural gas (see for example U.S. Pat. No. 5,496,589 Fong et al.) and heavy hydrocarbons such as pitch and atmospheric and vacuum residues (see for example Canadian Patent Application No. 2,060,108 Naber). Similarly, the oxygen and hydrogen which are combined with the carbon during the synthesis step to form methanol may be obtained from various sources. These include electrolysis, as well as the water gas shift reaction. For example, Avery and U.S. Pat. No. 5,416,245 (McGregor et al.) disclosed the use of electrolysis to provide hydrogen and oxygen. In the case of Avery, the oxygen is added together with steam to a gasifier to produce carbon monoxide and hydrogen for synthesis (column 4, lines 46-50).
Methanol is advantageous as a substitute fuel for gasoline as well as diesel fuel since it is a cleaner burning fuel (i.e. the fuel is converted to carbon dioxide and water with fewer by-products being produced). The reduced emissions associated with methanol will not favor its production unless methanol can be produced in a cost effective manner. In the retail marketplace, methanol must be priced competitively with gasoline and diesel fuel to be a commercial alternative fuel.
The advantage of methanol being a low polluting fuel will be reduced, or potentially lost, if the process for producing methanol has substantial emissions of greenhouse gases. Typical commercial processes which are in operation to date produce about 600 to 1200 pounds of carbon dioxide per ton of methanol produced. Therefore, while the methanol produced by these processes may be relatively non-polluting compared to gasoline and diesel fuel when it is combusted, when considered with the manufacturing process, the production and use of methanol may in fact be a substantial source of greenhouse gases.
In accordance with the instant invention, a process for the production of methanol is provided which has a reduced emission of carbon dioxide as a by-product of the manufacturing process. In particular, in accordance with the instant invention, a process for the production of methanol may result in the emission of only 240 pounds of carbon dioxide per ton of methanol produced and, preferably, 120 pounds or less of carbon dioxide per ton of methanol produced. In one embodiment of the present invention, the process has a net consumption of CO2. For example, the process may consume up to about 650 pounds of CO2 per ton of methanol produced.
In accordance with one aspect of this invention, a partial oxidation reactor is utilized to produce the synthesis gas which is then subjected to methanol synthesis to produce methanol and a tail gas stream. The tail gas stream has unreacted synthesis gases (including carbon dioxide) therein. A purge stream is removed to prevent the build up of inert gases therein. The remainder of the stream, or essentially all of the remainder of the stream, is recycled to the partial oxidation reactor. In this way, carbon dioxide, as well as carbon monoxide and methane, may be recycled through the system essentially to extinction except for the purge stream. The amount of greenhouse gases emitted by the process effectively depends upon the relative size of the purge gas stream to the recycle stream. The larger of the recycle stream, the smaller the greenhouse gases that are emitted. The recycle stream may comprise up to 95 weight percent, and preferably from 50 to 95 weight percent of the tail gas stream, based on the weight of the tail gas stream.
In order to reduce the size of the purge stream, the introduction of inert gases into the system is reduced. To this end, the oxygen which is used in the partial oxidation reactor preferably comprises essentially pure oxygen. In prior art processes, air or oxygen enriched air is utilized. This results in the introduction of substantial quantities of nitrogen. Not only does this result in the need to increase the size of the process equipment to have the same through put of methanol, but it also requires a larger purge stream and the consequential emission of additional greenhouse gases.
Typically, methanol production processes utilize reformers to provide additional hydrogen to the synthesis gas to obtain the desired stoichiometric of hydrogen to CO and CO2. Reformers, such as steam reformers, require the introduction of substantial quantities of water into the process and may result in the production of additional carbon dioxide at the expense of carbon monoxide formation. However, in one embodiment of the instant invention, a reformer is used to consume hydrogen in the conversion of CO2 to CO. This is the reverse of the current practice of operating a reformer. In accordance with another aspect of the instant invention, a hydrogen source other than reformers is utilized to adjust the hydrogen balance of the synthesis gas just ahead of the methanol reactor. Preferably, at least some of the hydrogen is obtained by electrolysis and more preferably essentially all of the hydrogen is obtained by electrolysis.
In a further preferred aspect of the instant invention, at least some of the electricity which is utilized in operating the electrolysis step is obtained as off-peak or valley power from a power grid. Typically, the power demand of a power grid varies throughout the day with the power demand from the grid being reduced at night when commercial and residential requirements are reduced. Not only may off-peak or valley power be obtainable at a reduced rate compared to peak demand time, but, in addition, the use of valley power may result in more efficient operation of power generating plants. For example, if it is necessary to reduce the electrical output of a power generation plant, then the efficiency of the plant may be reduced. Alternately, it may not be possible to reduce the power output of a generating plant thus resulting in the emission of greenhouse gases to produce power which is not required. Therefore, the use of valley power to run at least a portion of the electrolysis step may be highly beneficial. In fact, the oxygen and hydrogen produced by electrolysis such as at night may be stored in storage tanks so as to ensure a continuous supply of hydrogen and oxygen. Thus, if there is a power shortage during a peak demand period (e.g. during the day) then a continuous supply of hydrogen and oxygen may be provided. In this way, the feed of raw materials to produce a synthesis gas may be leveled to ensure a uniform continuous supply. In a further alternate embodiment, the electricity may be generated by running a fuel cell in reverse (i.e. a fuel cell may be operated to utilize an energy source such as electricity to produce hydrogen and oxygen).
Another advantage of the instant invention is that the amount of hydrogen which is produced may in fact exceed the amount of hydrogen required to produce the desired stoichiometric balance of the synthesis gas which is fed to the methanol synthesizer. Accordingly, the process may in fact also produce hydrogen as a valuable commercial product.
Accordingly, in accordance with this invention there is provided a process for the production of methanol comprising:
(a) feeding an amount of a hydrocarbon feedstock and an amount of an oxygen feedstock to a partial oxidation reactor to produce a partial oxidation reactor effluent comprising hydrogen, carbon monoxide and carbon dioxide;
(b) electrolyzing water to produce hydrogen and oxygen and recovering at least a portion of the hydrogen to produce a hydrogen stream;
(c) adding an amount of a hydrogen feedstock, at least a portion of which is obtained from the hydrogen stream, to the partial oxidation reactor effluent to produce a synthesis gas stream having a predetermined ratio of hydrogen to carbon monoxide;
(d) subjecting the synthesis gas to methanol synthesis to produce a methanol product stream and a tail gas stream;
(e) separating the tail gas stream into at least two streams comprising a purge stream and a recycle stream, the recycle stream comprising a substantial portion of the tail gas stream; and,
(f) recycling the recycle stream to the partial oxidation reactor.
In one embodiment, the process further comprises reforming the partial oxidation reactor effluent prior to the hydrogen addition step to convert at least some of the carbon dioxide to carbon monoxide. Optionally, a carbon dioxide feed stream may be provided.
In another embodiment, the process further comprises the step of recovering at least a portion of the oxygen produced by electrolyzing water to produce at least a portion of the oxygen feedstock.
In another embodiment, the process further comprises the step of adjusting the amount of the oxygen feedstock to the amount of the hydrocarbon feedstock fed to the partial oxidation reactor such that the partial oxidation reactor effluent contains some unoxidized hydrocarbon feedstock. The partial oxidation reactor effluent may contain up to about 10 wt % of the unoxidized hydrocarbon feedstock based on the weight of the partial oxidation reactor effluent and, preferably, the partial oxidation reactor effluent contains less than about 4 wt % of the unoxidized hydrocarbon feedstock based on the weight of the partial oxidation reactor effluent.
In another embodiment, the process further comprises the step of adjusting the amount of the oxygen feedstock to the amount of the hydrocarbon feedstock fed to the partial oxidation reactor such that the synthesis gas which is subjected to methanol synthesis is essentially free of oxygen.
In another embodiment, the synthesis gas which is subjected to methanol synthesis has a ratio of hydrogen minus carbon dioxide mole fraction to carbon dioxide plus carbon monoxide mole fraction of from about 1:1 to about 3:1.
In another embodiment, the synthesis gas, which is subjected to methanol synthesis has a ratio of hydrogen minus carbon dioxide mole fraction to carbon dioxide plus carbon monoxide mole fraction is about 2:1.
In another embodiment, the tail gas stream contains nitrogen and the method further comprises separating at least a portion of the nitrogen from the waste gas stream such that the purge stream is nitrogen rich and the recycle stream is a nitrogen reduced waste gas stream.
In another embodiment, a membrane separator is used to separate the tail gas into the nitrogen reduced waste gas stream and the nitrogen rich purge stream.
In another embodiment, the process further comprises combusting the nitrogen rich purge stream to produce energy.
In another embodiment, the combustion of the purge stream produces heat that is used to preheat at least one of the feedstocks of the partial oxidation reactor.
In another embodiment, the combustion of the purge stream produces electricity that is preferably used to electrolyze water.
In another embodiment, the partial oxidation reactor produces waste heat and the waste heat is used to generate electricity.
In another embodiment, the electrolysis is conducted by running a fuel cell in reverse.
In another embodiment, essentially all of the hydrogen and the oxygen is obtained by electrolysis.
In another embodiment, at least a portion of electricity used to electrolyze the water is valley power.
In accordance with another aspect of the instant invention, there is provided a process for the production of methanol comprising:
(a) feeding an amount of a hydrocarbon feedstock and an amount of an oxygen feedstock to a partial oxidation reactor to produce a partial oxidation reactor effluent comprising hydrogen, carbon monoxide and carbon dioxide;
(b) adding an amount of a hydrogen feedstock to the partial oxidation reactor effluent to produce a synthesis gas stream having a predetermined ratio of hydrogen to carbon monoxide; and,
(c) subjecting the synthesis gas to methanol synthesis to produce a methanol product stream and a tail gas stream wherein reformation is not used to provide hydrogen as a product.
In one embodiment, the process further comprises the step of recycling a portion of the tail gas stream to the partial oxidation reactor.
In another embodiment, the process further comprising the step of withdrawing a purge stream from the tail gas stream and recycling essentially the remainder of the tail gas stream to the partial oxidation reactor.
In accordance with another aspect of the instant invention, there is provided a process for the production of methanol comprising:
(a) feeding a hydrocarbon feedstock to a partial oxidation reactor to produce a synthesis gas comprising hydrogen, carbon monoxide and carbon dioxide;
(b) subjecting the synthesis gas to methanol synthesis to produce a methanol product stream and a tail gas stream;
(c) separating the tail gas stream into at least two streams comprising a purge stream and a recycle stream, the recycle stream comprising a substantial portion of the tail gas stream; and,
(d) recycling the recycle stream to the partial oxidation reactor.
In another embodiment, the tail gas stream contains nitrogen and step (c) comprises subjecting the tail gas stream to a separation process such that the recycle stream is nitrogen reduced and the purge stream is nitrogen rich.
In accordance with another aspect of the instant invention, a process for the production of methanol comprising:
(a) electrolyzing water to produce hydrogen and oxygen and recovering at least some of the hydrogen to produce a hydrogen stream and recovering at least some of the oxygen to produce an oxygen stream;
(b) feeding an amount of a hydrocarbon feedstock and an amount of an oxygen feedstock, at least a portion of which is obtained from the oxygen stream, to a partial oxidation reactor to produce an effluent gas stream comprising hydrogen, carbon monoxide and carbon dioxide;
(c) adding an amount of a hydrogen feedstock, at least a portion of which is obtained from the hydrogen stream, to the partial oxidation reactor effluent to produce a synthesis gas having a predetermined ratio of hydrogen to carbon monoxide;
(d) subjecting the synthesis gas to methanol synthesis to produce a methanol product stream and a tail gas stream:
(e) recycling a portion of the tail gas stream to the partial oxidation reactor; and,
(f) combusting the purge stream to obtain energy wherein reformation is not used to provide hydrogen as a product.
In one embodiment, the process further comprises combusting the nitrogen rich purge stream to produce energy.
In another embodiment, the combustion of the purge stream produces heat that is used to preheat at least one of the feedstocks of the partial oxidation reactor.
In another embodiment, the combustion of the purge stream produces electricity that is preferably used to electrolyze water.
In another embodiment, the partial oxidation reactor produces waste heat and the waste heat is used to generate electricity.
In another embodiment, the electrolysis is conducted by running a fuel cell in reverse.
In accordance with another embodiment of the instant invention, a process for the production of methanol comprises:
(a) feeding an amount of a hydrocarbon feedstock and an amount of an oxygen feedstock to a partial oxidation reactor to produce a partial oxidation reactor effluent comprising hydrogen, carbon monoxide and carbon dioxide;
(b) electrolyzing water to produce hydrogen and oxygen and recovering at least a portion of the hydrogen to produce a hydrogen stream;
(c) reacting carbon dioxide with hydrogen to produce carbon monoxide; and,
(d) subjecting a methanol synthesis gas obtained from the partial oxidation reactor effluent, at least a portion of the hydrogen stream and carbon monoxide produced by step (c) to methanol synthesis to produce a methanol product stream and a tail gas stream.
In one embodiment, the process as further comprises separating the tail gas stream into at least two streams comprising a purge stream and a recycle stream, the recycle stream comprising a substantial portion of the tail gas stream; and recycling the recycle stream to the partial oxidation reactor.
In another embodiment, the partial oxidation reactor effluent is fed to a reformer to produce a reformed synthesis gas and at least a portion of the hydrogen stream is combined with the reformed synthesis gas to produce the methanol synthesis gas.
In another embodiment, the process further comprises combining a carbon dioxide feedstock with the partial oxidation reactor effluent to produce a carbon dioxide rich synthesis gas stream and feeding the carbon dioxide rich synthesis gas stream to the reformer to produce a reformed synthesis gas.
In another embodiment, at least a portion of the hydrogen stream is combined with the reformed synthesis gas to produce the methanol synthesis gas.
In another embodiment, wherein at least a portion of the hydrogen stream is introduced to the reformer or a feedstream to the reformer.